1. Field of the Invention
This invention is directed to an improvement in the treatment of spent Kraft wood pulping liquors whereby sulfur contained in the coke resulting from the treatment is recovered as hydrogen sulfide which is in turn used to produce one of the pulping chemicals, Na.sub.2 S.
2. Description of the Prior Art
In commonly assigned copending application, U.S. Ser. No. 149,672, filed June 3, 1971 now abandoned is disclosed and claimed a process wherein spent alkaline Kraft pulping liquors are flowed to an absorbing zone and extraneous sulfur dioxide is added thereto to lower the pH of the liquors to below 7 and preferably between 2 and 5 prior to coking them in the liquid phase under autogenous pressure in a coking zone thereby forming coke, gases including H.sub.2 S and an aqueous effluent.
The coke is separated from the effluent in a separating zone and burned to produce the above-mentioned sulfur dioxide. Lime and H.sub.2 S are added to this effluent in a second absorbing zone to reconstitute Kraft cooking liquor.
Examination of the table appearing below, shows that liquid phase coking of the liquor after reducing its pH is considerably more efficient in terms of lowering the COD of the coker effluent. This is consistent with the higher coke yield and also with the higher gas yield. The sulfur content of the coke from the low pH run is much higher. The gas composition is markedly changed with the bulk of the sulfur from the low pH run appearing as H.sub.2 S. This is desirable in Kraft pulping because H.sub.2 S is a useful makeup chemical for production of Na.sub.2 S, one of the pulping chemicals.
Various techniques have been employed for reducing the sulfur content of petroleum coke. Thus U.S. Pat. No. 2,768,939 is a particular variation of the Esso fluid coking in which the hot petroleum coke is contacted with hydrogen at 1200.degree.-1400.degree. F. to remove sulfur from the coke. Petroleum coke is extremely dense and long times, 12 hours, are needed to reduce the sulfur content from 7 percent of 2 percent at a processing temperature of 1300.degree. F. U.S. Pat. No. 2,721,169 claims an improved method of reducing the sulfur content of petroleum coke by preoxidizing the coke with air (oxygen) followed by hydrogen treatment. The purpose of the air pretreatment is to form pores in the dense coke particles thereby improving the accessibility of the sulfur to hydrogen attack.
With the present invention only 2 hours of processing at 1350.degree. F. are needed to reduce the sulfur content from 14 percent to 2.14 percent. The properties of a fluid coke are compared below with the properties of the solids recovered from coking a Kraft spent liquor.
______________________________________ Coked Solids Petroleum from Kraft Sample Fluid Coke Liquor ______________________________________ Particle Density, g./cc 1.4 Less than one.sup.(1) Carbon, wt. % 89.1 51.2 Sulfur, Wt. % 6.0 14.0 Ash, wt. % 0.16 13.7 Metals V,Ni,Fe,Si Na,Ca Combustion Will support com- Will burn bustion only in readily the presence of oil. ______________________________________ .sup.(1) Fluffy solid that readily floats on water. Petroleum coke is a dense carbon containing material containing no oxygen while Kraft coked solids are estimated to contain at least 15 per cent oxygen. Accordingly, different techniques are needed to reduce the sulfur content in each case.
______________________________________ LIQUID PHASE COKING OF SPENT KRAFT LIQUOR 550.degree. F, 1100 psig, 2 HOURS Kraft Liquor with Kraft Liquor pH Adjusted pH 12.1 To 4.8 with SO.sub.2 ______________________________________ COD of Charge 195.6 g COD/ liter 195.6 g /COD /Liter Liquid COD of 85.9 g COD/liter 30.4 g COD/liter -of Coker Effluent COD Reduction % 56 84.5 Washed dry coke Yield wt. % 3.12 5.9 Wt. % Sulfur 2.07 14.0 Produced gas Yield wt. % 0.53 2.81 Analysis Mo.sup.1. % Dimethyl Sulfide 38.7 0.5 Methyl 18.0 2.4 mercaptan Carbon 8.75 61.0 dioxide Hydrogen 1.45 32.5 sulfide Ethane 1.0 0 Methane 6.1 0.7 Hydrogen 26.0 3.0 ______________________________________